The value returned is as follows:
MTI4OTMxODIgIA==
while the actual value is 12458

The LDAP-utitilities are not internationalized and so return always the
Base64-encoded value as saved in the server. Base64-encoding is for
numbers usually not necessary. How is the attribute "slot" defined?

The particular value quoted above "MTI4OTMxODIgIA==" has two trailing
spaces when decoded. That's the reason why ldapsearch displayed it in
base64 format, to ensure that the spaces don't get lost. If you got rid
of the trailing spaces, then it would just display it without base64
encoding.